Ashing techniques

Ashing methods and materials used for ashing.

1. Methods and techniques in Ashing By SELVAPRAKASH N 2015008040

2. Introduction The ash content   is a measure of the total amount of minerals present within a food, whereas the mineral content   is a measure of the amount of specific inorganic components present within a food, such as Ca , Na, K and Cl. Etc…

3. Reasons for determination of ash & mineral content Nutritional labeling Quality Microbiological stability Nutrition Processing

4. What is ash? Ash is the inorganic residue remaining after the water and organic matter have been removed by heating in the presence of oxidizing agents, which provides a measure of the total amount of minerals within a food. Analytical techniques for providing information about the total mineral content are based on the fact that the minerals (the analyte ) can be distinguished from all the other components (the matrix) within a food in some measurable way.

5. Methods of ashing Dry ashing Wet ashing

6. Sample preparation Normally ,1-10g of samples are used in the analysis of ash content . Solid foods are finely ground and then carefully mixed to facilitate the choice of a representative sample. S amples that are high in moisture are often dried to prevent spattering during ashing

7. Sample preparation High fat samples are usually defatted by solvent extraction, as this facilitates the release of the moisture and prevents spattering. contamination of samples by minerals in grinders, glassware or crucibles which come into contact with the sample during the analysis . For that reason , it is recommended to use deionized water when preparing samples . Samples are selected at random from large group of samples.

8. Dry ashing Dry ashing procedures use a high temperature muffle furnace capable of maintaining temperatures of between 500 and 600  o C. Water and other volatile materials are vaporized and organic substances are burned in the presence of the oxygen in air to CO 2 , H 2 O and N 2 . Most minerals are converted to oxides, sulfates, phosphates, chlorides or silicates .

9. Dry ashing M inerals are converted to oxides, sulfates , phosphates, chlorides or silicates. M ost minerals have fairly low volatility at these high temperatures, some are volatile and may be partially lost,  e.g.,  iron, lead and mercury . If an analysis is being carried out to determine the concentration of one of these substances then it is advisable to use an alternative ashing method that uses lower temperatures

10. Procedure for ashing Take a finely weighed ground sample in a silica porcelain crucible. Take a weight crucible with sample. Keep the sample over the flame for churring . After churring,keep the sample in muffle furnace at 500-550 c for 3-4 hours. Carefully remove the silica crucible from muffle furnace. Keep crucible in desiccator for cooling, after cooling take a final weight.

11. Ash calculation The food sample is weighed before and after ashing to determine the concentration of ash present. The ash content can be expressed on either a  dry  or  wet   basis.

12. Preparation of ash solution Step 1: Pour the ash in conical flask. Step 2: Add 1:1 HCL and Water Step 3: Approximately add 50 to 60 ml Step 4: K eep water bath in 30 mins Step 5: Cool into room temperature Step 6: Filter through whatman no1 filter paper Step 7: Rinse with small amount of hot water Step 8: Make upto 100 ml /250 ml using water

13. Step Temperature and time Device Drying Less than 110 °C 1-2 hours Hot air oven Charring <250°C 1-2 h Hot plate or gas stove Combustion 450-500°C 12-20 h (min4-5 hour) Mufffle furance Ash dissolution 20-100°C 0.5-1 h Hot plate

14. Containers used for ashing Quartz Pyrex Porcelain steel Platinum

15. Why mostly use porcelain crucible ? I nexpensive to purchase. U sed up to high temperatures (< 1200 o C ) Easy to clean. R esistent to acids

16. Advantages of Ashing High efficiency of decomposition Large sample weights Low consumption of reagents Low cost Low safety risks

17. Disadvantages Losses of some analytes (volatile elements Hg, Ti, Se, As, P…) cannot be used for determination of all elements Sample contamination Inconvenient for liquid samples Time-consuming procedure

18. Wet ashing Wet ashing is primarily used in the preparation of samples for subsequent analysis of specific minerals .   It breaks down and removes the organic matrix surrounding the minerals so that they are left in an aqueous solution .  A dried ground food sample is usually weighed into a flask containing strong acids and oxidizing agents ( e.g .,mostly   nitric, perchloric and/or sulfuric acids) and then heated.

19. Wet ashing principle Heating is continued until the organic matter is completely digested , leaving only the mineral oxides in solution. The temperature and time used depends on the type of acids and oxidizing agents used.  Typically , a digestion takes from 10 minutes to a few hours at temperatures of about 350 o C. The resulting solution can then be analyzed for specific minerals.

20. Extraction of the Analyte T he sample is not soluble in water and must be treated with acids or mixtures of acids to facilitate solubilization . The type of acid treatment must be given careful consideration, since particular acids may or may not oxidize the sample, and may be incompatible with certain elements

21. Examples sulfuric acid –barium containing samples HCL -silver or lead samples The choice of acid is also restricted by sample volatilization . e.g ., HCL should be avoided in samples containing arsenic .since this is more volatile as a trichloride

22. Types Wet chemical digestions utilizing various mineral acids(eg.HCL,HNO3,HF,H2SO4,etc.),hydrogen peroxide and other liquid reagents is carried out in either an open system ,that is under closed system open acid digestion closed acid digestion(under pressure)

23. Reagents for wet digestion

24. Chemistry of acid digestion Organic samples are generally decomposed into carbon dioxide with the aid of oxidizing acids primarily nitric acid and reagents ( primarily hydrogen peroxide) and completely mineralized. selection of reagents depending upon type of material to be analyzed. This is an exothermic reaction,so precautions required Samples can safely decomposed by slow heating rates or by heating to different temperature levels

25. single acid digestion Sample pretreatment must be followed. e.g .Ceramic samples –high temperature-HCL Hot HNO3-not suitable for Al&Cr (surface oxide) Hot H2SO4-mostly high BP (340 c)-oxidize HClO4- potent oxidizing agent- dehydrates &oxidizes organic samples very efficiently- iron alloys and stainless steel- Percholorate salts HF -weak , non oxidizing acid that is particularly useful for dissolving silicate samples

26. Chemistry of solvents HNO3: Oxidizing agent Frequently mixed with Hydrogen peroxide or HCL,HF,H2SO4

27. Chemistry of solvents

28. Chemistry of vessel materials used in wet acid digestion Naturally the employed vessels must be resistant to the acids&reagents being used . Glass (except or solutions conaining HF) ,Teflon materials are used.

29. Structure of containers

30. Closed wet aid digestion(under pressure) A modern method used in analytical laboratories most often Closed decomposition vessels – made of teflon (PTFE) – temp. Limit approx. 240 °c made of silica glass – used up to 320 °c Sample weight 0.2-1 g of dry matter (a large sample weight → explosion!) Reagents: HNO3,HNO3+H2O2 , HNO3+HF (ONLY IN PTFE) Heating – conventional heating of the whole vessel – microwave heating → easy control of the process

31. Pressure decomposition Safety: sample oxidation produce a lot of gaseous products (nitrogen oxides, CO2 , water vapour ) → pressure increases (units to tens of mpa ); Vessels are placed in massive jackets made of steel or PEEK Sensors monitor pressure and optionally both pressure and temperature inside the vessels When pressure exceeds the chosen limit microwave heating is stopped; in the case of explosive reaction a controlled expansion occurs Efficiency of decomposition: at t < 200 °C partial decomp . Only (pressure solubilization ); final t > 280 °C is needed to achieve the complete destruction of organic matter (necessary when a voltammetric method of determination is to be used) Duration (decomposition and cooling): 20 min to 2 h

32. Steel pressure digestion (12 samples)

33. Advantages Almost no losses of analytes Minimal contamination Much safer than decomp . Using an HCLO4 containing mixture No difficulties associated with the use of H2SO4 Low consumption of acids Fast decomposition (microwave) → automation

34. Disadvantages Expensive equipment Low sample weight → a sample has to be completely mixed Lower efficiency at t < 200 °C Resulting solution contains HNO3 → interference in hydride generation determination of as, se, sn etc.

35. Open acid digestion Closed acid digestion Maximum temperature limited by the solutions boiling point Maximum temperature :260-300 c Permits large sample weight Permits minimum quantity of samples High acid consumption Low acid consumption Loss of volatile elements ( e.g.Hg,Pb ) Comparatively less Contamination risk Less contamination risk l Digestion time taken high Microwave digestion 20 -60 mins

36. Other sample prepration methods Microwave digestion Ultraviolet digestion (photolysis) Vapor phase acid digestion

37. Microwave digestion Samples in digestion equipment heated by microwaves directly by the absorption of microwaves under closed condition. Once the set point temperature attains decomposition starts same rate. Nearly take 20-40 mins .

38. Microwave digester

39. Advantages Determine parameters Measurement is easy Contamination is less Sensor systems No man power and additional things

40. Vapor phase acid digestion Recent tech .developed with in 40 years Mathusiewice -vapor phase attacking the dissolution &decomposition of inorganic and organic materials determination of trace elements(digestion - gasphase ) HF &HNO3(half) acid vapor as digestion agent Zilbershtein -dissolve si and con impurities on a PTFE sheet.Residue and PTFE sheet placed in graphite electrode.spectroscopic analysis-failur e

41. Sample placed in PTFE beaker.-perforated PTFE mounted. Kept above the level of liquid HF.(Closed system) HF vapor pass.sample have HCLO4&HNO3 Thomas and smythe explained vapor phase oxidation This tech introduced by wooley . Low temp to high temp (110-250) Device consist-airtight PTFE vessel with 2 concentric chambers.

42. Ultraviolet digestion Liquids and slurries solids decomposed by UV lights in the presence of small amounts of H2O2 &HN03 or peroxothiosulphate ( bev,waste water ,sewage treatment plants ) Digestion vessel keep in closet –UV lamp Digestion mechanism –conversion of OH- radicals from both water and hydrogenperoxide that is initialized by UV These radicals able oxide to CO2 and water. Not fully oxidized.partly like chlorinated phenols , nitrophenols,hex.chl.benzene .

43. Thanking you…………